Solar power generation (photovoltaic power generation) in which a light energy is converted into an electric energy by a photoelectric transfer effect has been extensively used as a means for attaining a clean energy. With the recent enhancement in photoelectric transfer efficiency of solar batteries, solar power generation systems have been installed even in a large number of individual houses. In order to employ these solar power generation systems as a practical power source, there has been used a solar battery module having a plurality of solar battery cells which are electrically connected in series to each other.
Since the solar battery module is used under high-temperature and high-humidity environmental conditions for a long period of time, a film for a backsheet of solar batteries is also required to exhibit a long-term durability. For example, there has been proposed the technique in which a fluororesin-based film is used as the film for a backsheet of solar batteries (Patent Document 1). In the thus proposed technique, it is described that the fluororesin-based film is previously subjected to heat treatment to reduce a shrinkage factor of the fluororesin-based film, so that there can be obtained the effect of preventing deterioration in properties of the film including weather resistance and water resistance when subjected to vacuum lamination with ethylene vinyl acetate (hereinafter occasionally referred to merely as “EVA”) as a sealing material as well as the effect of enhancing a yield of the film. However, since the fluororesin-based film is expensive, there tends to arise such a problem that a solar battery module produced using such a fluororesin-based film is also expensive.
Conventionally, there has been proposed the technique in which a polyester-based film is used as the film for a backsheet of solar batteries. However, as is known in the art, when using the polyester-based film under high-temperature and high-humidity environmental conditions, there tends to occur such a problem that the polyester-based film suffers from hydrolysis at an ester bond moiety in a molecular chain thereof, so that mechanical properties of the polyester-based film tend to be deteriorated. In consequence, in view of such a case that the polyester-based film is used outdoors over a long period of time (for example, over 20 years) or under high-humidity environmental conditions, there have been made various studies for suppressing occurrence of hydrolysis of the polyester.
It is known that the rate of hydrolysis of a polyester becomes higher as a content of a carboxyl end group in a molecular chain of the polyester is increased. Therefore, there has been proposed the technique in which by adding a compound capable of reacting with a carboxylic acid, the amount of a carboxyl group being present in a terminal end of a molecular chain of the polyester is reduced to thereby enhance a hydrolysis resistance of the polyester (Patent Documents 2 and 3). However, these compounds added tend to induce gelation of the polyester upon melt extrusion step or a material recycling step in a film formation process to thereby cause generation of foreign matters in the film, resulting in high burdens on environments and high product costs.
In addition, aside from the conventional concept that the solar batteries are disposed on roofs, there is recently an increasing demand for “solar batteries of a see-through type” which are designed to imagine a window glass. The solar batteries of such a type are capable of generating an electric power while ensuring a good outside view from an inside thereof. The film for a backsheet of the solar batteries of this type is required to have not only a long-term durability but also a low haze.
Upon polycondensation of a polyester as a raw material of the polyester film, antimony trioxide has been extensively used as a catalyst for the polycondensation reaction because it is inexpensive and exhibits an excellent catalytic activity. If the antimony trioxide is used as a main component of the polycondensation catalyst, i.e., added in such an amount as is capable of exhibiting a practical performance of the polymerization catalyst, the antimony trioxide tends to be reduced upon the polycondensation reaction, which results in production of metallic antimony particles. As a result, in the subsequent melt extrusion step for forming a film, the metallic antimony particles tend to be aggregated together and therefore present in the resulting film in the form of black foreign matters having a size of 20 to 50 μm. That is, there tends to remain such a problem that these aggregated metallic antimony particles inhibit penetration of light through the film and increase a haze of the film.
Even though a filter is used upon the melt extrusion step in order to remove the aggregated metallic antimony particles, it may be difficult to completely remove the particles because the aggregated metallic antimony particles tend to still pass through the filter while being deformed.
In addition, upon polycondensation of the polyester as a raw material of the base polyester film, a germanium compound has also been used as the polymerization catalyst. However, the germanium compound is very expensive, and therefore it may be difficult to generally use the germanium compound.
In order to solve the above conventional problems, there has been proposed the technique in which contents of a titanium compound and a phosphorus compound in a film are limited to specific ranges to reduce internal foreign matters in the resulting film (Patent Document 4). However, in the above proposed technique, any oligomers which tend to be generated upon a melt polymerization step for production of a polyester have not been taken at all into consideration. That is, in the above technique, the oligomers tend to be generated inside of the resulting film or on a surface of the film, and it may be therefore difficult to obtain a film having a low haze.